The objectives of this study are 1) to elucidate the essentiality of essential fatty acids through study of the effects of positional and geometric isomerism upon metabolism of unsaturated acids and 2) to study quantitative aspects of essential fatty acid requirements. The first objective will be approached through comparative studies of the relative utilizations of positional cis and trans isomers of octadecenoic acid. This work will also speak to the current issue of the nutritional value of trans fatty acids present in hydrogenated fat. The utilization of the isomers will be studied using several mammalian enzyme systems. The possible inhibitory effects of trans and acetylenic acids upon enzyme systems will be studied in vitro. The second objective will involve experiments to test the interaction of other nutrients with the metabolism of essential fatty acids, and their possible effect upon quantitative requirements of essential fatty acids.